Ice is the solid form of water. The phase transition occurs when liquid water is cooled down to 0°C (273K, 32°F) at standard atmospheric pressure. Ice can be formed at higher temperatures in pressurized environments, and water will remain a liquid or gas until -30°C at lower pressures. Ice formed at high pressure has a different crystal structure and density than ordinary ice.
Ice, water and water vapour can coexist at the triple point, which for this system is 273.16K at a pressure of 611.73 Pa.
An unusual feature of ice frozen at a pressure of one atmosphere is that the solid is less dense than liquid water (10% less). This is due to hydrogen bonds between the water molecules, which line up molecules less efficiently (in terms of volume) when water is frozen. The result of this is that ice floats on liquid water, an important factor in Earth's climate.
Everyday ice and snow is Ice Ih, or hexagonal ice. Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases. Only a little less stable (metastable) than Ih is cubic structure ice (Ic). But cooling Ih causes a different arrangement to form in which the protons move, XI.
With both cooling and pressure more types exist, each being created depending on the phase diagram of ice. These are II, III, V, VI, VII, VIII and X. With care all these types can be recovered at ambient pressure. The types are differentiated by their crystalline structure, ordering and density. There are also two metastable phases of ice under pressure, both fully hydrogen disordered, these are IV and XII. Ice XII was discovered in 1996. As well as crystalline forms solid water can exist in amorphous states as amorphous solid water (ASW), low density amorphous ice (LDA), high density amorphous ice (HDA), very high density amorphous ice (VHDA) and hyperquenched glassy water (HGW).
Note: In addition to these real variants of ice, a fictional "ice-nine" featured in Kurt Vonnegut's novel Cat's Cradle. Real Ice-IX does not have the properties of Vonnegut's fictional ice-nine.
Rime is a type of ice formed by fog freezing on cold objects. It contains a high proportion of trapped air, making it appear white rather than transparent, and giving it a density about one quarter of that of pure ice.
Ice can also form icicles, similar to stalactites in appearance, as water drips and re-freezes.
Clathrate hydrates are forms of ice that contain gas molecules trapped within its crystal lattice.
See also - iceberg, dry ice, rusticle, crystal, properties of matter, water.Types of ice
Ice-free harbors
For harbors near the poless, being ice-free is an important advantage, ideally all-year round. Examples are Murmansk (Russia), Petsamo (Russia, formerly Finland) and Vardø.
